The differential pressure bonding process can be used to bond together layered metallic materials to produce high-temperature panels and shells for use in modern aircraft. Such panels and shells can include an open cell or “honeycomb” metallic core and metallic face sheets covering the opposed faces of the core. The differential pressure bonding process generally includes applying a pressure differential across an assembly of metallic layers to simultaneously compress and bond the materials together at an elevated temperature. A bonding material disposed between the metallic layers bonds the sheets together such as by liquid interface diffusion bonding or brazing. The metallic layers can include titanium or Inconel® face sheets, titanium or Inconel® honeycomb cores, and the like.
The differential pressure bonding process was developed to provide a method of compressing an assembly of layered materials together as the materials are heated to a bonding temperature. The term “differential pressure” refers to a difference in pressures across a plurality of layered materials that acts to press the layered materials against a forming surface as the materials are heated and bonded together. The forming surface can be a surface of a plate, a mandrel, a die, or other tool having a surface profile corresponding to a desired shape of the bonded structure. A method of bonding metallic panels by the differential pressure bonding process is described in U.S. Pat. No. 5,199,631, assigned to Rohr, Inc., for example.
To produce a metallic shell structure having a simple cylindrical or conical shape using the differential pressure bonding process, metallic components to be bonded (such as a titanium or Inconel® honeycomb core and titanium or Inconel® face sheets) can be positioned within a one-piece mandrel having a cylindrical or conical inner surface that corresponds to a desired outer shape of the cylindrical or conical shell. A thin layer of a suitable bonding material is applied between the surfaces of the materials to be bonded. As the assembled layers of materials and bonding material are heated to the bonding material's bonding temperature, a pressure differential is applied across the layered materials such that the layers are forced against the contoured inner surface of the mandrel. When the layered materials and bonding material are heated to the bonding temperature, the bonding material melts and fuses the layered materials together. Because the layered materials have been forced against the contoured inner surface of the mandrel by the applied pressure differential during heating of the bonding material, the outer surface of the resultant shell has a shape corresponding to the mandrel's cylindrical or conical inner surface. Once the shell has cooled, the simple cylindrical or conical shape of the one-piece mandrel permits the bonded shell and mandrel to be separated by longitudinally separating the shell and the mandrel.
Though the differential pressure bonding process described above can be used to produce shell structures having relatively simple cylindrical, conical, or substantially conical shapes, producing high temperature axisymmetric shells having complex curvatures by such a process presents special challenges. As used herein, the term “complex curvature” means having concave and/or convex curvatures wherein at least one intermediate diameter of the shell is either larger or smaller than both diameters at the ends of the shell. As described above, after bonding layers of a cylindrical or conical shell, the shape of the mandrel and the shell permits the shell and mated mandrel to be separated by pulling the bonded shell and mandrel apart in a longitudinal direction. For shells having non-cylindrical and non-conical shapes and substantial convex and or concave curvatures, however, the bonded shell is entrapped within the mandrel such that the substantially rigid shell cannot physically be extracted from the mandrel in a single piece. One solution to this problem is to produce the shell in a plurality of generally conical or generally cylindrical sections using the process described above, and joining the formed sections together end-to-end using connecting hardware and/or by welding to form a complete shell. Such a multi-section shell necessarily includes at least one girth seam around the shell's circumference.
High-temperature axisymmetric shells like those described above can be used to form portions of modern aircraft engines. For example, such a high-temperature metallic shell can form at least a portion of an engine's exhaust nozzle center plug. FIG. 1 shows one example of a typical engine exhaust nozzle center plug 10. The center plug 100 includes a centerbody shell 102 and a tail cone 104. The centerbody shell 102 is joined to the tail cone 14 along a circumferential girth seam 106. The centerbody shell 102 has an axisymmetric convex curvature with a maximum diameter corresponding to highlight 108 (indicated by a dashed line in FIG. 1). As discussed above, existing methods of bonding a metallic shell having such complex curvatures using the differential pressure bonding process dictate that the centerbody shell 102 must be constructed in at least two sections separated at a girth seam 103 corresponding to the shell's maximum diameter. Thus the centerbody 102 must be constructed of a forward shell portion 112A having a first generally conical shape, and an aft shell portion 112B having a second generally conical shape. The generally conical shapes of the forward and aft shell portions 112A, 112B permits the shell portions to be longitudinally separated from a one-piece mandrel having a corresponding substantially conical shape after bonding.
One goal in the design of some modern aircraft engines is to minimize the noise emitted by the engines, especially during approach and take-off conditions. Accordingly, modern aircraft engines can include noise-attenuating panels and noise-attenuating shells designed to at least partially dissipate noise generated by an engine's combustor and rapidly rotating fan and rotor blades. As shown in FIG. 2, an engine's noise emissions can be at least partially reduced by providing an outer skin 129 of the center plug's centerbody shell 112 with perforations 120 that permit acoustic communication with underlying open cells 122 of the shell's honeycomb core 124. A non-perforated inner skin 127 covers the inside of the open-cell core 124. Such an arrangement is known to dissipate acoustic energy via Helmholtz resonance. The depths of the open cells 122 can be sized to dissipate acoustic energy having a targeted frequency or range of frequencies. In order to maximize a structure's noise-attenuating capability, the percentage of the structure's exterior surface area associated with a perforated outer skin 129 and underlying open cells 122 should be maximized in order to maximize the number of active resonant cavities.
As discussed above, existing methods of bonding high-temperature shells having complex curvatures using the differential pressure method dictate that such shells must be constructed in at least two sections, and joined together end-to-end along at least one circumferential girth seam. Conventional methods of joining separate shell sections along a girth seam can reduce the surface area of a shell that is available for acoustic treatment, because skin perforations and cell cavities proximate to the seam often are blocked by connecting hardware and/or by welds commonly used to join such shell sections. In addition connecting hardware used to join shell sections increases the number of parts, and adds to the overall weight of the structure. Accordingly, there is a need for a method of bonding metallic layers of an axisymmetric shell having complex curvatures by the pressure differential bonding process such that shell is produced in a single piece. By producing the shell in a single piece, perforation and cell blockages associated with girth seam connections can be eliminated, and the external surface area of the shell available for acoustic treatment is maximized. In addition, producing a shell in a single piece can reduce the number of parts and the total weight of the structure, and can reduce overall production time.